1. Field of the Invention
This invention relates to an exposure meter in which it is possible to use either a photovoltaic element such as a photodiode or a photoconductive element such as a CdS element as a photoreceptor, and it is possible to indicate as an indication exposure determining factors such as apex indication Bv of the brightness of the object B, apex indication Tv of the shutter speed T and apex indication Av of the aperture size, and it is also possible to indicate alignment of marks (generally, of fixed points).
2. Description of the Prior Art
There have been a number of proposals regarding exposure meter circuits employing a photovoltaic element such as a photodiode or a photoconductive element such as a CdS element. It has not been proposed, however, to provide an exposure meter circuit which is capable of using either a photovoltaic element or a photoconductive element due to the difference in physical properties therebetween.
In the photovoltaic type photoreceptor, when used with inverse bias, the photocurrent i.sub.p is EQU i.sub.p = K.sub.1 B.sup..gamma..sbsp.1 1.
where B is the brightness of the object, K.sub.1 is a proportional constant and .gamma..sub.1 is a constant, and in the photoconductive type photoreceptor, the resistance Rcds is EQU Rcds = K.sub.2 B.sup.-.sup..gamma..sbsp.2 2.
where K.sub.2 is a proportional constant and .gamma..sub.2 is a constant. That is, there is a difference between the above two types of photoreceptors that in the photovoltaic element the amount of light is converted to a current and in the photoconductive element it is converted to a resistance. Further, in the former, the constant .gamma..sub.1 is .gamma..sub.1 = 1 when a photodiode is used, and in the latter, the constant .gamma..sub.2 is .gamma..sub.2 = 0.5-1 when CdS is used. In other words, in a photovoltaic element, the photocurrent is proportional to the brightness of the object B and in the photoconductive element the resistance is proportional to 1/2 -1/1 power of the brightness of the object B. In addition, in contrast to that the photocurrent handled in the photoconductive element is several tens of pA to several tens of .mu.A, the photocurrent handled in the photovoltaic element is several .mu.A to several mA. Owing to these differences between the photovoltaic element and the photoconductive element, it has been very difficult to provide an exposure meter circuit which is capable of using both types of photoreceptors.